Development of FePt–Si–N nanocomposite thin films for magnetic recording

Applied Surface Science 300 (2014) 124–128

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Development of FePt–Si–N nanocomposite thin films for magnetic recording Jing Liu a , Y.P. Zeng a , H.Y. Yu a , D.L. Jiao a , Z.G. Zheng a , Z.W. Liu a,∗ , G.Q. Zhang b a b

School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China Science and Technology on Advanced High Temperature Structural Materials Laboratory, Beijing Institute of Aeronautical Materials, Beijing 100095, China

a r t i c l e

i n f o

Article history: Received 10 January 2014 Received in revised form 4 February 2014 Accepted 5 February 2014 Available online 15 February 2014 Keywords: FePt Magnetic recording Magnetron sputtering Nanocomposite magnetic film

a b s t r a c t Nanocomposite FePt–Si–N thin films consisting of magnetic L10 FePt nanograins and non-magnetic Si–N matrix for high density recording medium are developed in this work. The (Fex Pt100-x )100-y Siy –N films (x = 50–65 and y = 0–10) were fabricated on Si (100) substrates by dc reactive magnetron sputtering followed by vacuum annealing. The maximum value of coercivity around 15.3 kOe was obtained in the film with atom ratio Fe:Pt = 55:45. To achieve a high coercivity, the concentrations of Si and N also have to be properly controlled. Doping Si–N improves the coercivity of FePt film through weakening the exchange coupling interaction. Si and N also play an important role in reducing average grain size of the magnetic particles and smoothing the surface of the films. Transmission electron microscopy demonstrated the nanogranular structure consisting of FePt nanoparticles embedded in the amorphous Si–N matrix.

Introduction Recently, much effort has been made to explore magnetic recording media for the extremely high density recording. The ideal materials for these media must have high coercivity, small and uniform grain size, and high thermal stability. The CoPt and FePt alloys with magnetocrystalline anisotropy constant Ku , about 5 × 107 erg/cm3 and 7 × 107 erg/cm3 , respectively, have been considered as the potential candidates [1–3]. However, to maintain the high thermal stability of the FePt or CoPt films and to eliminate the medium noise, the magnetic grain interactions have to be reduced [4,5]. For this purpose, a nonmagnetic matrix can be introduced into the film to form the nanocomposite structure, by which the magnetic grains can be isolated with each other. During last years, much research work have been focused on the FePt films with various non-magnetic matrices, such as Ag [6], Al2 O3 [7], B2 O3 [8], SiO2 [9], BN [10], Si3 N4 [11], and C [12]. Inspired from the concept of FeCo–S–N thin films with self-assembled nanoparticle structure [13], our previous investigations have shown that the grain size of the FePt can be minimized by dispersing magnetic FePt grains into an amorphous nonmagnetic Si–N matrix [14]. In this work, we developed the FePt–Si–N films with optimized composition and enhanced coercivity. Based on the prepared (Fex Pt100-x )100-y Siy –N thin films with x = 50–65, y = 0–10 and various N contents, the

∗ Corresponding author. E-mail address: [email protected] (Z.W. Liu). http://dx.doi.org/10.1016/j.apsusc.2014.02.020 0169-4332/© 2014 Elsevier B.V. All rights reserved.

© 2014 Elsevier B.V. All rights reserved.

effects of Fe/Pt atom ratio and Si–N addition on the structure, magnetic properties and exchange coupling of the films are reported here. The microstructure and Si–N distribution in the film are also discussed with the help of the transmission electron microscopy.

Experimental The FePt–Si–N films were fabricated on Si (100) substrates by dc reactive magnetron sputtering at room temperature. The film thickness was fixed at 200 nm. A mosaic target of a Pt disk symmetrically distributed with fan shaped Fe and rectangle Si (5 × 10 × 0.5 mm3 ) chips was used in this work to deposit the thin films with expected compositions. The purity of the Fe, Pt, and Si employed were higher than 99.99%. The base pressure of the sputter chamber was controlled at 6.0 × 10−5 Pa and the argon pressure was fixed at 0.7 Pa. An argon and nitrogen gas mixture was used as the sputtering gas, and the N content was adjusted by changing the nitrogen partial pressure. The as-deposited film was encapsulated in a quartz tube, annealed in vacuum at 700 ◦ C for 1 h, and then quenched in ice water. The structure of the films was examined by conventional X-ray diffraction (XRD) with Cu K␣ radiation and atomic force microscope (AFM). The microstructures were observed by transmission electron microscopy (TEM), and magnetic properties of the films were measured by vibrating sample magnetometer (VSM) with a maximum applied field of 80 kOe. The composition and thickness of the films were determined by an energy dispersive X-ray spectrom-

J. Liu et al. / Applied Surface Science 300 (2014) 124–128

(b)

Si 2p 20% N 10% N Si 2p3

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N 1s

Intensity(a.u.)

Intensity(a.u.)

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20% N 10% N 0% N

0% N

100 102 104 106 108 110

Binding Energy(eV)

392

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Binding Energy(eV)

Fig. 1. XPS high resolution spectra for the (Fe60 Pt40 )90 Si10 –N films with various N contents: (a) Si element, (b) N element.

eter (EDS), X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM).

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Results and discussion

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Si content (at.%) Fig. 3. Variation of average grain size with Si content for (Fe60 Pt40 )100-y Siy –N (y = 0–10) films.

eventually confirm that the increased non-magnetic phase distribution between FePt magnetic particles can weaken the exchange coupling and improve the coercivity. Fig. 3 shows the variation of average grain size with Si content for the annealed (Fe60 Pt40 )100-y Siy –N films with y varied from 0 to 10. The nitrogen partial pressure is fixed at 10%. The average grain size is estimated using Scherrer formula according to (111) peak width from the XRD results. For pure FePt film, the grain size is around 40 nm. When Si–N is added into FePt film, the FePt grain size decreases from about 25 to 16 nm with increasing Si content from 5 to 10 at.%. The result shows that the growth of magnetic FePt grains can be suppressed by the Si addition. Based on above results, the next work mainly focused on the films with 10 at.% Si content. Fig. 4 shows the in-plane coercivities of the (Fex Pt100-x )90 Si10 –N films with x varied from 50 to 65, and the nitrogen partial pressure is fixed at 10%. When the Fe content increases, the coercivity increases first and reaches its maximum

(Fe60Pt40)90 Si10-N

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(Fe60Pt40)93 Si7 -N

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(Fe60Pt40)95 Si5 -N

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HC (kOe)

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HC (kOe)

Grain size(nm)

In our previous paper [14], the Fe, Pt, and Si contents in the FePt–Si–N films were characterized by EDS and the films were denoted as (Fe100-x Ptx )100-y Siy –N based on their compositions. The phase structure of the films was confirmed as fct-FePt phase and the amorphous Si–N. To identify the chemical state of Si and N elements in the film, the XPS high resolution spectra of these two elements for (Fe60 Pt40 )90 Si10 –N films are obtained, as showed in Fig. 1. The results clearly indicate that Si and N were successfully doped into the film. The change of nitrogen partial pressure has little influence on the intensity of Si 2p peak (Fig. 1(a)), confirming the constant concentration of Si. Fig. 1(b) shows that the intensity of N 1s peak gradually increases with the introduction and increase of nitrogen in the sputtering gas. Both figures indicate that Si and N are indeed in the amorphous state. Fig. 2 shows the variations of in-plane coercivity with N partial pressure for various (Fe60 Pt40 )100-y Siy –N films with y varied from 5 to 10. For these three series of films with various Si contents, the coercivity increases first and then decreases with the increasing nitrogen partial pressure. The maximum coercivity of the films with various silicon contents were obtained at different nitrogen partial pressures. For the (Fe60 Pt40 )90 Si10 –N film containing relatively high silicon content the maximum HC value was obtained at a high nitrogen partial pressure, while the films with less Si content possess the highest coercivity at the lower nitrogen partial pressure. The results suggested that, to achieve a high coercivity in the FePt–Si–N films, a proper proportion of Si and N contents be necessary. For the films with similar N content, the coercivity increases with the increasing silicon content. The films with 10 at.% Si have relatively higher coercivity values than others. The results

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N partial pressure(%) Fig. 2. Variations of HC with nitrogen partial pressure for (Fe60 Pt40 )100-y Siy –N films with various Si contents.

0 50

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Fig. 4. Variation of coercivity for (Fex Pt100-x )90 Si10 –N films with x varied from 50 to 65 and prepared with nitrogen partial pressure of 10%.

J. Liu et al. / Applied Surface Science 300 (2014) 124–128

of 15.3 kOe at x = 55. The reason is that the ordering degree of Ferich films is higher than that of Pt-rich samples. In other words, the ordered L10 phase is easier to be formed in Fe-rich films. The coercivity of the FePt films is mainly determined by the ordering degree [15]. Generally, the higher is the ordering degree, the larger the coercivity is. Therefore, the coercivity of the film with 55 at.% Fe is much higher than that with 50 at.% Fe. The result also shows that further increasing Fe content leads to a dramatic drop of coercivity. The coercivity is only 8.6 kOe when Fe content up to 65 at.% due to the formation of soft magnetic Fe3 Pt phase. Therefore, the atomic ratio of Fe to Pt is very important for preparing high coercivity (Fex Pt100−x )–Si–N films. The optimum Fe/Pr ratio for the maximum value of coercivity is around x = 55/45. Henkel Plots is suggested to be effective for analyzing the exchange interactions between the grains based on the relationship ıM(H) = Md (H)–[1–2 Mr (H)], where Mr (H) and Md (H) are defined as the remanent magnetization after applying a field H on a thermally demagnetized sample and a reverse field on a previous saturated sample. The positive value of ␦M is primarily caused by intergrain exchange coupling. Fig. 5 shows the Henkel plots of Fe45 Pt55 and (Fe55 Pt45 )90 Si10 –N (20% nitrogen partial pressure) films [14]. Lower positive peak for Si–N doped film indicates a reduced exchange coupling between neighboring FePt grains. The exchange decoupling is certainly induced by Si–N phase located between FePt particles. Also, generally, the peak of the ␦M plot locates in the vicinity of the coercivity. Fig. 5 demonstrates again that Si and N doping greatly improve the coercivity of the FePt film, which should results from the reduced exchange coupling interaction. This has been confirmed by our previous report [14]. Fig. 6 shows AFM images of Fe55 Pt45 Si–N films with various silicon contents. The nitrogen partial pressure is 10% for Fig. 6(b) and (c). The average roughness (Ra) of the film surface decreases with increasing Si content. The value of Ra decreases from 1.7 nm for Fe55 Pt45 film to 0.8 nm for films with 10at.% silicon content. There-

0.3

Fe55Pt45 0.2

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δM

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0.0 -0.1 -0.2 0

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H (kOe) Fig. 5. Henkel plots of Fe55 Pt45 and (Fe55 Pt45 )90 Si10 –N prepared with nitrogen partial pressure of 20%.

fore, the addition of Si and N plays an important role in suppressing grain growth and smoothing the surface of the films. Both of them are beneficial for the high-density magnetic recording medium. Fig. 7 shows TEM results of (Fe55 Pt45 )90 Si10 –N film annealed at 700 ◦ C for 1 h. The selected area electron diffraction (SAED) pattern in Fig. 7(a) reveals that annealing at 700 ◦ C for 1 h results in the formation of ordered L10 -FePt phases. The superlattice diffraction ring (201) from the L10 -FePt phase can be clearly observed in addition to fundamental diffraction rings. Si–N compounds diffraction spots are also not found in the diffraction pattern, confirming the amorphous structure of Si–N. Fig. 7(b) shows a high-resolution TEM micrograph of the film (in-plane observation). The grain size is approximately 16 nm, roughly agreeing with the XRD calculation. The crystalline phase of L10 -FePt and a part of the grain boundary can be observed. The Si–N amorphous phase is found between the FePt phases. The interplanar spacing of the crystallized phase

Fig. 6. AFM images of FePt films (a) Fe55 Pt45 , Ra = 1.7 nm, (b) (Fe55 Pt45 )95 Si5 –N, Ra = 1.2 nm, and (c) (Fe55 Pt45 )90 Si10 –N, Ra = 0.8 nm.

J. Liu et al. / Applied Surface Science 300 (2014) 124–128

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Fig. 7. High-resolution TEM image and electron diffraction pattern of (Fe55 Pt45 )90 Si10 –N film.

Fig. 8. TEM cross-sectional bright field image of (Fe55 Pt45 )90 Si10 –N films (a) and its partial energy spectrum scanning images for Fe element (b) and Pt element (c).

˚ which in accordance with (111) showed in Fig. 8b is about 2.2 A, interplanar spacing of L10 -FePt phase. The result indicates that the growth of the films is along the (111) plane and the similar result has been observed by XRD analysis as (111) plane shows the strongest intensity. Fig. 8 shows TEM cross-sectional bright field image of (Fe55 Pt45 )90 Si10 –N films (a) and its partial energy spectrum scanning images for Fe and Pt elements. From Fig. 8a, the nanocomposite structure of the film is demonstrated. In Fig. 8(b) and (c), the dark areas stands for Si–N phase while relatively bright areas means the Fe and Pt elements, i.e., FePt phase. The very few dark areas observed in the EDS spectra may be due to the relatively large thickness of the TEM sample. Nevertheless, the results, thus, confirmed that introduction of Si and N in FePt film led to the formation of nanocomposite structure.

Conclusions The (Fex Pt100-x )100-y Siy –N thin films have been developed in this work. The magnetic properties and microstructures of the films were studied in detail. The XRD and high-resolution TEM analysis reveal that (111) texture is dominant in FePt–Si–N thin films. The VSM measurement revealed that Fe-rich films are beneficial to the formation of ordered L10 phase and the maximum value of coercivity around 15.3 kOe is obtained in the films with atom ratio Fe:Pt = 55:45. The FePt–Si–N films with a proper proportion of Si and N contents exhibit a high coercivity. Doping Si–N improves the coercivity of FePt film through weakening the exchange coupling interaction. The addition of silicon and nitrogen play an important role in reducing average grain size of the magnetic particles and smoothing the surface of the films. TEM images also reveal that the Si–N compound existed in films is in amorphous state.

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The nanocomposite structure of the films has been verified. It is expected that this structure can be contributed to the new generation magnetic recording medium. Acknowledgments This work is supported by the Program for New Century Excellent Talents in University (Grant No. NCET-2011-0156) and the Fundamental Research Funds for the Central Universities, SCUT (Grant No. 2012ZZ0015). References [1] M. Hong, K. Hono, M. Watanabe, Microstructure of FePt/Pt magneticthin films with high perpendicular coercivity, J. Appl. Phys. 84 (1998) 4403–4409. [2] J. Christodoulides, Y. Zhang, G. Hadjipanayis, C. Fountzoulas, CoPt and FePt nanoparticles for high density recording media, IEEE Trans. Magn. 36 (2000) 2333–2335. [3] Y.-N. Hsu, S. Jeong, D.E. Laughlin, D.N. Lambeth, Effects of Ag underlayers on the microstructure and magnetic properties of epitaxial FePt thin films, J. Appl. Phys. 89 (2001) 7068–7070. [4] J.-G. Zhu, Transition noise properties in longitudinal thin-film media, IEEE Trans. Magn. 29 (1993) 195–200.

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